1. Field of the Invention
This invention generally relates to a gallium nitride (GaN) compound semiconductor and to gallium nitride semiconductor light emitting diodes (LEDs) or semiconductor laser diodes. More particularly, the invention relates to LEDs or semiconductor lasers that include within their active region a layer of indium rich nitride. The presence of such an active layer within such a device can allow an LED or laser to output red light. This invention also relates to a method of manufacturing an active layer comprising an indium rich region and LEDs and lasers including such an active layer.
2. Description of the Related Art
Gallium nitride compound semiconductors and more specifically indium-aluminum-gallium nitride (InAlGaN) compound semiconductors are direct transition or direct bandgap semiconductors capable of efficiently producing high levels of luminosity. The bandgap of InAlGaN compound semiconductors can be adjusted between about 1.89 eV and about 6.2 eV by varying the composition of these semiconductors. Since the wavelength of output light varies with the bandgap of material within the active region for InAlGaN semiconductors, widely varying wavelengths of output light can be obtained by using different compositions of InAlGaN semiconductors in the active region of a light emitting device. Such devices are also considered to be good candidates for producing short wavelength light output. Few possibilities exist for producing short wavelength light output from a semiconductor device. Consequently, research has been directed to developing short and visible wavelength lasers and high luminosity light emitting devices employing gallium nitride compound semiconductors.
InAlGaN compound semiconductors are formed basically by combining in the deposition process the following binary compounds: gallium nitride (GaN), aluminum nitride (AlN) and indium nitride (InN). Gallium nitride is the most difficult of these materials to work with and has consequently been the subject of the most research. Gallium nitride has a high melting point of about 1700xc2x0 C. and nitrogen has an extremely high equilibrium vapor pressure at the temperatures appropriate to growing gallium nitride, which makes growing single crystal gallium nitride difficult. Because of the difficulty of growing single crystal gallium nitride, advanced thin film growth techniques have been used, including hydride vapor phase epitaxial (VPE) growth or metal organic chemical vapor deposition (MOCVD). MOCVD has been studied extensively and can be applied to grow single crystal films of the binary compounds. Adding indium or aluminum during the growth of gallium nitride grows the ternary compound indium gallium nitride (InXGa1xe2x88x92XN) or aluminum gallium nitride (AlYGa1xe2x88x92YN). By using these ternary materials in the heterostructure active region of a light emitting device, the efficiency of light generation can be increased over simpler light emitting device structures. Furthermore, by forming a double heterostructure active region to more effectively confine both the injected carriers and the light generated in the active region, a high luminance semiconductor LED or a short wavelength semiconductor laser diode can be obtained.
By increasing the mole fraction X of indium in the compound, the bandgap of InXGa1xe2x88x92XN can be changed from about 3.4 eV, the bandgap of GaN, to about 1.89 eV, the bandgap of InN. Consequently, light-emitting devices can use InXGa1xe2x88x92XN in their active regions to produce visible light output. The ternary compound InXGa1xe2x88x92XN is obtained by simultaneously depositing gallium nitride and indium nitride while setting the deposition gas mixing conditions to achieve the desired composition.
The differences in the typical growth conditions for gallium nitride and indium nitride make growth of the ternary compound difficult. For example, higher quality crystalline gallium nitride films result from growth temperatures of over 1000xc2x0 C., while a much lower temperature is appropriate to grow indium nitride crystals. For this reason, InXGa1xe2x88x92XN, with a high mole fraction X, is typically grown at a temperature well below the growth temperature for GaN (Appl. Phys. Lett. 59(18), p.2251, Oct. 28, 1991). Research on the application of InXGa1xe2x88x92XN as the active layer for an LED emitting blue light, X=0.2, and green light, X=0.45, has been reported and such an LED has almost reached commercial production. (Jpn. J. Appl. Phys., Vol. 34 (1995), pp. L1332-L1335, part. 2, No. 10B, Oct. 15, 1995). However, if the proportion of indium in InXGa1xe2x88x92XN is increased for the purpose of emitting a larger-wavelength red light, the quality of crystals of InGaN degrades and the output intensity and conversion efficiency declines correspondingly. The difficulty of making InGaN of selected compositions has prevented from being successfully commercialized for longer-wavelength light emitting devices. InGaN LEDs with an emission wavelength of 594 nm are reported to have been developed, and this apparently is the longest wavelength ever achieved so far(Jpn. J. Appl. Phys., Vol. 37 (1998), pp. L479-L481, part. 2, No. 5A, May 1, 1998).
Indium aluminum nitride (InYAl1xe2x88x92YN), formed from the constituents of InN and AlN, has also been studied and found to have potential for use as the active region material for an optical device emitting visible light. There is, however, a mismatch between the lattice constants of InN and AlN that makes it difficult to form heterostructures between these materials and that causes the ternary compound crystals to have internal strains. Also, there is a gap between the equilibrium vapor pressures for the two binary compounds InN and AlN. At the high temperature at which AlN crystals are typically grown, InN evaporates. This introduces further difficulty in growing InYAl1xe2x88x92YN crystalline structures, as illustrated in T. Matsuoka, Proc. of ICN ""97 at 20 (Oct. 1997, Tokushima, Japan).
U.S. Pat. No. 5,780,876 to T. Hata describes the use of an evaporation preventing layer to prevent indium evaporation from an indium gallium nitride compound semiconductor. For example Al0.05Ga0.95N can be deposited at a reduced temperature over an In0.2Ga0.8N compound semiconductor layer to prevent indium from evaporating from the In0.2Ga0.8N active layer during subsequent processing. If not prevented, evaporation of indium from the active region leads to degradation of the interface between the active layer and the upper cladding layer. Such evaporation makes it difficult to control the crystal composition and thickness of the active layer.
Other efforts to facilitate the growth of heterostructures of InAlGaN have not yet been successful. For example, research on the microwave enhanced metal organic vapor phase epitaxial growth method, MOVPE, which would grow InN crystalline structures at lower temperatures, has been under way. At this time, the method has not been validated and the InN grown using this technique does not have satisfactory properties for efficient light emission.
Nitride compound semiconductors including indium have so far failed to realize their high potential for use as materials for the active regions of semiconductor devices emitting red light. Consequently, these materials have not yet achieved practical commercial use, due in large part to the fact that the technology for growing high quality crystal films of the indium-rich nitride compounds has not been established.
xe2x80x9cIt is an object of the present invention to provide an indium-rich nitride compound for use as an active layer in an optical device to permit the device to emit red light having a peak wavelength greater than or equal to 630 nm, to provide LEDs and semiconductor lasers employing such nitride compound, and to provide a method for manufacturing such nitride compound.
A preferred aspect of the present invention provides a nitride compound semiconductor with a mixture of areas including areas where the composition is indium-rich and is expressed as InX1AlY1Ga1xe2x88x92X1xe2x88x92Y1N (0 less than X1xe2x89xa61, 0xe2x89xa6Y1 less than 1) and other areas where the composition is aluminum-rich and is expressed as InX2AlY2Ga1xe2x88x92X2xe2x88x92Y2N (0 less than X2 less than X1xe2x89xa61, Y1 less than Y2xe2x89xa61).
Another aspect of the present invention provides a nitride compound semiconductor light emitting device having a stack of layers including an active layer, the composition of each layer being expressed as InXAlYGa1xe2x88x92Xxe2x88x92YN (0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61). The composition of the active layer preferably comprises areas where the composition is indium-rich and is expressed as InX1AlY1Ga1xe2x88x92X1xe2x88x92Y1N (0 less than X1xe2x89xa61, 0xe2x89xa6Y1 less than 1) and other areas where the composition is aluminum-rich and is expressed as InX2AlY2Ga1xe2x88x92X2xe2x88x92Y2N (0 less than X2 less than X1xe2x89xa61, Y1 less than Y2xe2x89xa61).xe2x80x9d
Still another aspect of the present invention provides a method of manufacturing a nitride compound semiconductor light emitting device having a stack of layers. A first layer is grown at a first temperature to obtain an incomplete crystalline structure including both indium and aluminum with the composition expressed as InXAlYGa1xe2x88x92Xxe2x88x92YN (0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61). The method grows a cap layer on the first layer to cover the first layer at a second temperature lower than the first temperature. The first layer is heated to a third temperature greater than the first temperature to cause the incomplete crystalline structure of the first layer to crystallize and to create separate areas of differing compositions. In preferred embodiments of the method, the material of the cap layer is selected as to be heat stable during the heating step.
xe2x80x9cYet another aspect of the present invention provides a method of manufacturing a nitride compound semiconductor light emitting device having a stack of layers. The method grows a first layer at a first temperature to obtain an incomplete crystalline structure, including both indium and aluminum with the composition of the first layer expressed as InXAlYGa1xe2x88x92Xxe2x88x92YN (0 less than X less than 1, 0 less than Y less than 1). The method then grows a cap layer on the first layer to cover the first layer, with the growth proceeding at a second temperature below the first temperature. The first layer is heated at a third temperature above the first temperature to cause the incomplete crystalline structures of the first layer to form an active layer. The material of the cap layer is selected to be stable during the heating step.
Another aspect of the invention provides a method of manufacturing a nitride compound semiconductor light emitting device with a stack of layers having a cladding layer. A first layer is grown on the cladding layer at a first temperature to obtain an incomplete crystalline structure including both indium and aluminum with the composition of the first layer expressed as InXAlYGa1xe2x88x92Xxe2x88x92YN (0 less than X less than 1, 0 less than Y less than 1). The method grows a cap layer on the first layer to cover the first layer, with the cap layer grown at a second temperature below the first temperature. The first layer is heated at a third temperature above the first temperature to cause the incomplete crystalline structure of the first layer to form an active layer. The material of the cap layer is selected to be stable during the heating step.xe2x80x9d